The present invention relates to an optical pickup for a system for recording information on an optical disc such as a write-once Digital Versatile-Recordable (DVD-R), and reproducing the information therefrom, and more particularly to an optical element provided in the pickup.
There has been known the use of a write-once Compact Disc-Recordable (CD-R) where information can be recorded in large quantities. An optical pickup for recording and reproducing information on and from the CD-R is described with reference to FIG. 6.
Referring to FIG. 6, the conventional optical pickup comprises a laser diode 1 which emits a laser beam. The laser beam is shaped into parallel rays by a collimator lens 2 and split into three beams by a grating 3, the beams of which are a center zeroth-order diffraction main beam and first-order diffraction sub-beams deflected in positive and negative directions from the center zeroth-order main beam. The beams are focused on a track of a disc 9 through a beam splitter 40, quarter-wave plate 5 and an objective 6. The beam splitter 40 has a polarizing film 41 which is adapted to transmit P (parallel) polarized light and to reflect S (senkrecht) polarized light. A shaping prism 42 is further provided on the beam splitter 40.
Since the recording and reproducing performance of the CD-R depends on the wavelength of the laser beam, the ordinary laser diode 1 for the CD-R is constructed to emit a laser beam having a wavelength of about 780 nm. The laser beam is generally a linearly polarized light beam which is propagated in the TE mode, that is, in a direction parallel to an active layer (not shown) of a resonator of the laser diode 1. Such a polarized beam forms a far-field pattern of an elliptic shape having the minor axis thereof in the polarizing direction. When such an elliptic beam, the polarizing direction of which coincides with that of the P polarized light with respect to the polarizing film 41, passes through the shaping prism 42 of the beam splitter 40, the component in the direction of the minor axis is enlarged at a predetermined rate thereby forming a circular pattern. Namely, the P polarized component of the laser beam, converted into the circular beam is transmitted through the beam splitter 40 and applied to the quarter-wave plate 5. At the quarter-wave plate 5, the P polarized light becomes circularly polarized light.
The polarized main beam and the sub-beams are applied to the recording surface of the disc 9 and reflected therefrom. The reflected three beams are applied to the quarter-wave plate 5 through the objective 6, so that the beams are converted from the circularly polarized beams to the linearly polarized beams. The linearly polarized beam is the S polarized beam, the polarizing direction of which is rotated 90 degrees with respect to beams applied from the beam splitter 40 to the quarter-wave plate 5. Since the beams passes through the quarter-wave plate 5 twice before and after reflection, the P polarized light is converted to S polarized light. The beams which originally consisted of P polarized light are converted into beams of S polarized light. Accordingly, when the reflected beams enter the beam splitter 40, the beams are reflected from the polarizing film 41 thereof, and applied to detecting elements of a photodiode 8 as a photodetector through a condenser lens 7.
The photodiode 8 generates signals in accordance with the detected reflected main beam and sub-beams to reproduce the information and to operate a focus servo system and a track-following servo system at recording and reproducing of information by the main beam. Namely, a focus error signal is generated in accordance with the astigmatic method or the Foucault method, and a tracking error signal is generated based on the sub-beams in accordance with the well-known differential push-pull method or the three-beam method.
Meanwhile, the development of the DVD-R is in progress. In the DVD-R, the recording density is extremely increased, thereby enabling to record a huge quantity of information. In such a high-density optical disc, it is necessary to decrease the diameter of the laser beams as the recording density is increased. Namely, the wavelength of the beams is shortened to less than 660 nm, for example, about 635 nm. Hence a laser diode which emits a laser beam of shorter wavelength than that of the laser diode 1 in FIG. 6, hereinafter called a short wavelength laser diode, must be used instead of the laser diode 1.
However, contrary to the laser diode 1, hereinafter called the long wavelength laser diode, the short wavelength diode emits a linearly polarized light in a so-called TM mode wherein the light is polarized in a direction perpendicular to the active layer of the resonator of the diode. Such a light beam forms a far-field pattern of an elliptic shape having the major axis in the polarizing direction.
If such a laser diode is to be adopted in the optical pickup shown in FIG. 6, and the laser beam is applied to the shaping prism 42 so that the polarizing direction coincides with the P polarized light direction of the polarizing film 41, each of the elliptic shapes is enlarged in the direction of the major axis thereof by the shaping prism 42. Therefore, the beams cannot be shaped into circles.
On the other hand, if the laser beams are to enter in the direction where the polarizing direction thereof coincides with that of the S polarized light, since the reflectance of the S polarized wave does not become completely zero percent as shown by the characteristics of the polarizing film 41 shown in FIG. 7, there occurs a problem that the utilizing efficiency of the light beams emitted from the light source is decreased. Namely, due to the operation of the polarizing film 41, the light from the light source (S polarized light) is not transmitted one hundred percent. FIG. 7 shows the characteristic of reflectance, that is equivalently, the transmittance thereof, with respect to the angle of incidence into the polarizing film 41. The incident angle of 56 (.theta..sub.P) degrees corresponds to the angle of polarization where the reflectance of the P polarized light is zero percent, that is, where the transmittance is one hundred percent. The beam splitter 40 is so designed that the light beam enters the polarizing film 41 at the angle of polarization.
If the beam splitter 40 is to maintain its function, and at the same time serve to shape the far-field pattern of the beams to form a circle, the optical path starting from the short wavelength laser diode and leading to the shaping prism 42 must be appropriately designed. However, the designing of the path is troublesome, and the path becomes complicated, thereby enlarging the optical pickup.